The audible range of many hearing impaired individuals is compressed to a limited dynamic range of sound. For such individuals, soft sounds may be inaudible while loud sounds are heard at the same sound level as persons with normal hearing.
Electronic devices which employ automatic gain control (AGC) circuitry to compensate for these hearing deficiencies are well known. These devices are often designed to compress the sound level delivered to their users by providing greater amplifier gain for soft sounds and reduced gain for loud sounds.
The response time of AGC's is commonly characterized by two parameters, the attack and release time, which are defined by the American National Standards Institute (ANSI). The attack time is defined as the time between the input signal's abrupt increase from 55 to 80 Db and the point where the output level has decreased and stabilized to within 2 dB of the steady value for the 80 dB input sound pressure level (SPL). The release time is defined as the interval between the input signal's abrupt drop from 80 to 55 dB and the point where the signal has increased and stabilized to within 2 dB of the steady state value for the 55 dB input sound pressure level.
A common problem with AGC's employed in hearing aid compression systems is that no single choice of attack and release time adequately compensates for all signals. For instance, a circuit with both a fast attack and release time frequently causes audible "pumping" of the input signal. Conversely, too long a release time will produce audible gaps, especially if the input signal contains short transients resulting in long periods of reduced gain. Attack and release times of 10 ms and 200 ms, respectively, have been used in prior art hearing aids to minimize audible pumping of the input signal.
Various solutions attempting to solve this problem have been proposed. For example, U.S. Pat. No. 4,718,099 issued Jan. 5, 1988 to Hotvet discloses the use of peak detectors in a hearing aid circuit to provide faster attack and slower release times. Peak detectors can sense signal peaks, such as the breaking of glass or sounding of a horn, and quickly reduce amplifier gain. Moreover, peak detectors allow for setting a variable release time independent of the attack time and dependent upon the longevity of the input peak.
It is believed that the human ear does not respond as a pure peak detector, however. An article entitled "The Dynamics of Compression: Some Key Elements Explored", published in the November 1993 issue of The Hearing Journal explains that the human ear behaves more like an average detector. That is, the perceived loudness of a signal depends more on the signal's average level than it does on the signal's peaks. Accordingly, peak detection circuitry, which adjusts the loudness of an entire signal based on the signal's peaks, may not adequately compensate for hearing deficiencies as can average detectors.
Unlike peak detectors, most prior art average detectors have not provided separate attack and release times. Prior art average detectors which have attempted to implement independent attack and release times have failed, however, to adequately increase the intelligibility of normal speech.
As a general rule, consonants within spoken words provide more distinguishing information than vowels. Individuals with hearing deficiencies often have more difficulty hearing consonants than vowels. Such individuals therefore have difficulty understanding normal speech. This difficulty is due, in large part, to the fact that consonants are spoken at relatively higher frequencies and lower volumes than vowels, paralleling the range of audible sound frequently lost by hearing impaired individuals.
An ideal hearing aid therefore would accentuate consonants without also emphasizing vowels, thereby increasing the intelligibility of normal speech for hearing impaired individuals. Such an ideal aid should attack transitions from softer consonants to louder vowels quickly to de-emphasize vowels sounds within a word, and it should also release quickly at transitions from vowels to consonants to accentuate consonants. The desired increased speed of release must be balanced against avoiding pumping the input signal. Thus, in an ideal aid, only a portion of the gain should be restored quickly and the remainder should be restored at a slower rate. The ideal aid would react quickly enough to emphasize low volume, high frequency consonant sounds without amplifying them enough to cause pumping.
Prior art hearing aids that use average detection have not achieved this ideal response. Instead of first releasing a portion of gain quickly and then slowing down, prior art hearing aids release at a constant rate. Prior art hearing aids, thus, fail to adequately accentuate consonants and therefore fail to adequately increase the intelligibility of normal speech.
As stated previously, loud sounds are heard by many hearing impaired individuals at the same sound level as individuals with normal hearing. Hearing aid users will experience discomfort if sounds reaching their ears are amplified too much. Through signal compression, prior art hearing aids provide reduced gain for loud sounds. Even compressed sounds may be amplified to uncomfortably loud levels, however, depending upon both the listener and listening environment.
Prior art hearing aids provide adjustable volume controls to reduce or increase the overall system gain. Thus, if a user is in a loud environment or is overly sensitive to loud sounds, the user may turn down the hearing aid volume. By doing so, however, the user also changes the gain for soft sounds. By lowering the volume to avoid uncomfortably loud sounds, the prior art hearing aid user also sacrifices the gain necessary to hear soft sounds.
Since loud sounds are heard by many hearing impaired individuals at the same sound level as individuals with normal hearing, hearing aid users likely prefer to amplify loud sounds with unity gain. Prior art hearing aids can only provide unity gain for loud sounds at one particular volume control setting. The gain for soft sounds cannot be set independently.